Resident leukocyte populations and inflammatory response in the abdominal adipose tissue of the red seabream Pagrus major

Abstract

Adipose tissue is not only an energy depot but also an important endocrine and immunological tissues in mammals. However, the study of functions of fish adipose tissue, other than lipid storage, is considerably limited. In the present study, we investigated resident leukocyte populations in the visceral adipose tissue (VAT) of the red sea bream, Pagrus major, an important species of Japanese aquaculture with a well-developed VAT. We found that the tissue contains a significant number of leukocytes; it is mainly composed of eosinophilic granulocytes, macrophages, unclassified large cells, and a smaller number of lymphocytes. The VAT cellular composition is similar to that of peritoneal cells, but not of the peripheral blood leukocytes. Gene expression analysis also confirmed that all examined genes, including CD4, CD8α, membrane and secretory IgM heavy chains, macrophage-colony stimulating factor receptors (type 1 and 2), macrophage mannose receptor, MHC class II, and Toll-like receptors 2, 3, 5, and 9, were expressed in the VAT. Intraperitoneal injection of formalin-killed Vibrio anguillarum markedly upregulated the mRNA expression of IL-1β, IL-8, and IL-6 in the VAT within six hours. In addition, in vitro bacterial stimulation upregulated these cytokines. These results indicate that the VAT of P. major is an immune-competent organ that evokes an inflammatory response against intraperitoneally introduced bacteria.